1. Field of the Invention
The present invention relates to a two-dimensional absolute encoder and a scale therefor.
2. Description of the Related Art
Conventionally, an incremental encoder and absolute encoder are used for the purpose of measuring a stage position of a mechanical apparatus. Applications for incremental encoder towards machine tools and robot fields are limited since absolute position information cannot be obtained unless an origin is first detected. On the other hand, an absolute encoder does not require an origin to be detected since it can obtain absolute position information.
In order to obtain a two-dimensional absolute encoder in X and Y directions, it is common practice to use a method of respectively laying out and detecting two absolute linear encoders on orthogonal side surfaces of a stage. However, a two-dimensional scale is often required to be laid out on one surface such as an upper or lower surface of a stage and to detect that scale by an external detector in individual situations of apparatuses. Some methods of forming a two-dimensional scale pattern having absolute codes have been proposed. Japanese Patent Laid-Open No. 11-248489 discloses a method of embedding patterns used to specify absolute positions at equal intervals in a two-dimensional plane. A two-dimensional absolute encoder described in Japanese Patent Laid-Open No. 11-248489 encodes using black and white patterns by segmenting absolute position information to be embedded at one grid position into a plurality of regions (elements).
Japanese Patent Laid-Open No. 2008-525783 also discloses a method of embedding patterns used to specify absolute positions at equal intervals in a two-dimensional plane. In the method described in Japanese Patent Laid-Open No. 2008-525783, color patterns such as red, blue, and green patterns are laid out around measurement points prepared at equal intervals, and that layout state expresses absolute position information. However, a scale or detecting optical system using color information often requires high cost. A two-dimensional scale described in Japanese Patent Laid-Open No. 2004-333498 divides a two-dimensional plane into square grid regions at equal intervals, thereby giving quasi-random patterns respectively in X and Y directions. With the method described in Japanese Patent Laid-Open No. 2004-333498, specific regions of quasi-random patterns respectively given in the X and Y directions are clipped and simultaneously detected, and a two-dimensional absolute position is specified by combining correlation calculations based on the obtained two-dimensional image pattern. However, the method of designing the two-dimensional pattern described in Japanese Patent Laid-Open No. 2004-333498 is complicated.
In the two-dimensional absolute encoders described in these Japanese Patent Laid-Open No. 11-248489, Japanese Patent Laid-Open No. 2008-525783, and Japanese Patent Laid-Open No. 2004-333498, the resolving power of an imaging (image-taking or image-detecting) optical system is considered as a limit of resolution enhancement (high-resolution enhancement), and resolution enhancement comparable to an incremental encoder cannot be achieved. The two-dimensional absolute encoders described in Japanese Patent Laid-Open No. 11-248489, Japanese Patent Laid-Open No. 2008-525783, and Japanese Patent Laid-Open No. 2004-333498 require high cost since they require a two-dimensional light-receiving element array such as a CCD, marks with a complicated structure, and an image processing technique.